1. Field of the Invention
The present invention relates to apparatus for sensing power line voltage and current components of alternating current electric energy, and more particularly to novel sensors for sensing line currents and scaling such currents to produce corresponding analog output signals proportional to the line currents while compensating for thermal and magnetic field effects and minimizing phase errors between the sensed line currents and the analog output signals.
2. Description of the Prior Art
Many electrical and electronic devices such as, for example, induction and electronic type watthour meters for metering electric power and energy usage, require means for sensing line current components flowing in a conductor, and producing an output signal which is accurately proportional, over a large range of magnitudes, to the line current.
Induction type watthour metering devices for alternating current electrical energy measurement have been provided by producers for years, and have been used almost exclusively for measuring energy consumption by separate electrical energy users. Typically, watthour meters are used for indicating consumption in kilowatt hours. Induction type watthour meters typically have voltage and current coils and a rotating disc driven by fluxes from the two coils.
The current sensing circuit of a conventional induction watthour meter senses the current to be measured in a very direct manner. This sensing is done by wrapping the line current carrying conductor around an iron core to form a current coil to create a magnetic flux in the core which is used, in conjunction with flux from the voltage coil, to rotate the above mentioned disc at a rate proportional to electrical energy consumption. Current coil designs have been made using as many as 640 ampere turns around the core and which are capable of carrying up to 320 amperes.
Generally, if the magnitude of the line current to be measured is in excess of the above mentioned 320 amperes, or alternatively if the line conductor is at some high voltage potential with respect to ground, it is common practice, and frequently required, to interpose a current transformer between the watthour meter and the line conductor or current carrying circuit being metered. Such current transformers are usually rated at a nominal current in the secondary winding of 5 amperes, however, in many instances up to 20 amperes of secondary current may be used without exceeding the thermal rating of the transformer. A meter which is used to serve the above requirement is commonly referred to as a "transformer rated" watthour meter and typically has a maximum current coil rating of 20 amperes.
Large currents and voltages of the above type are suitable for induction type watthour meters. However, such large currents and voltages cannot be used in the design of a fully electronic watthour meter which uses electronic or integrated circuits for measurement and which are designed to operate at small signal levels many magnitudes below the line currents and voltages. The voltage and current inputs to the types of integrated circuits referred to are typically less than 5 volts or 5 milliamperes. Therefore, it is generally required that the sensors providing voltage and current responsive analog signals to the measuring circuits have large transformation ratios. In the case of current sensors, their output signals must be linear over a wide range in the magnitudes of the line current supplied to the sensors.
At a typical power utility customer location, sixty Hertz AC electric power is delivered at substantially constant line voltages in the range of 120-480 volts defining the line voltage components of the electric energy quantity to be measured.
On the other hand, line current which defines the current component of the electric energy to be measured, varies considerably. Typically, this variation is in a desired linear measurement range from one-half ampere to two hundred amperes or in a current range of approximately one to four hundred. Accordingly, conventional voltage transformer arrangements often can provide practical voltage sensing devices. However, current transformers receiving the aforementioned wide input variations and producing the required low level signal outputs often require structural arrangements which contribute to a substantial size and consequent increase in cost. It is well known by designers of such current transformers that the ampere turns of the primary and of the secondary must be equal. In addition, since maximum primary current levels may reach two hundred or more amperes, the primary and secondary winding sizes become large in order to produce the low level output signals required by the electronic measuring circuits. In view of the foregoing, it can be seen that a need exists for a current sensing apparatus for scaling a large range of currents (e.g. 20 to 200 amperes or more) to a suitable voltage and current level (e.g. 1-5 volts or 1-5 milliamperes) for input to the above measuring circuits. Such scaling necessitates a large scaling ratio of 100,000:1 or more. In addition, if adequate metering accuracy is to be achieved, the magnitude and the phase angle of an output signal from the sensing apparatus must be very accurate with respect to the magnitude and phase of the line current being applied to the apparatus. Furthermore, the sensing apparatus must be physically small enough to fit within the physical design constraints, or envelope restrictions, of a conventional induction type watthour meter in order to comply with industry standards and to permit direct interchange of an electronic meter in place of an induction meter.
In addition to the foregoing needs, the sensing apparatus must provide isolation between its output signal point and its point of connection to the line current conductor, which may be operating at high current levels and at voltage levels of 480 volts rms or above. Such isolation also dictates that high voltage transients on the line current conductor be suppressed to prevent such transients from getting through the sensing apparatus and affecting the output signal therefrom.
Another problem which arises in current sensors which use current coils or transformers is saturation of the magnetic core caused by substantially any d.c. component superimposed on the waveform of the a.c. current flowing in the line conductor. It is well known by meter designers that a small d.c. component may be present on the line conductor as a result of incidental half wave rectification caused by various electrical apparatus connected to the line conductor. Persons intent on committing meter fraud have also been found to deliberately insert much larger d.c. components on the line conductor to significantly affect the metering accuracy of the current sensor. As such, a need also exists for a current sensor design which cannot be compromised by the presence of any d.c. component on the line conductor.
Current sensors are also known to generate an external magnetic field, and to also be affected by incident magnetic fields from other sources (including adjacent current sensors such as are employed in polyphase watthour meters). Thus, a further need exists for a current sensor design which generates only a minimum external magnetic field and one which is essentially unaffected by incident magnetic fields from other sources. Such a current sensor is highly desireable in a watthour meter where multiple current sensors are operated in close proximity.
Many prior art current sensor and transducer designs are known using various techniques, attempting to fulfill and solve the above needs and problems such as disclosed in U.S. Pat. Nos. 4,182,982; 4,492,919; 4,496,932; 4,513,273; 4,616,174; and 4,749,940. U.S. Pat. Nos. 4,492,919 and 4,749,940 are assigned to the assignee of the present invention.
In attempts to solve the above mentioned problems and needs, the prior art contains several approaches for dividing a load current into two current paths to produce a sample current or voltage proportional to the load current.
Typical current divider techniques are disclosed in U.S. Pat. Nos. 4,182,982 and 4,492,919, wherein a current in a line conductor is split, or divided, into a main shunt path and a parallel auxiliary path. The auxiliary path contains a much smaller cross section than does the main shunt path and current through the divider path combination divides in substantially the ratio of the cross sections. A toroidal magnetic core with a winding of many turns is disposed about the auxiliary path. The auxiliary path thus forms a one-turn primary and the many turns about the toroidal core form a secondary winding. A current through the secondary is proportional to the current in the primary divided by the number of turns in the secondary. The techniques disclosed in these patents have several disadvantages. They use copper and thus suffer from reduced accuracy due to the substantial thermal coefficient of resistance of copper which may cause the resistance to change as much as 30 percent over the environmental temperature range to which watthour meters are exposed. In addition, it is difficult to obtain a sufficient current division to give the many orders of magnitude reduction (scaling) in output current or voltage compared to load current. Finally, these techniques are subject to output signal errors resulting from incidental magnetic fluxes surrounding the current divider and coil combination.
A further technique, disclosed in U.S. Pat. No. 4,496,932, employs two slits in a substantially flat and longitudinal conductor to accommodate a measurement conductor which is inserted between a pair of shunt conductors connected in parallel with the measurement conductor. The measurement conductor is deflected, first in one direction, and then in the other, to provide an opening or space for the passage of a one turn loop of magnetic core material between the shunt conductors and the measurement conductor. In one embodiment, the shunts and the measurement conductor are folded into a U-shape to align holes in each leg of the U. The one-turn of the loop of magnetic material is then passed through the aligned holes for receiving a sample of current produced by the presence of the slits and the measurement conductor when current is passed through the shunt conductors and the measurement conductor. A secondary winding of many turns on the core loop serves to provide an output signal. This device suffers from the presence of strong magnetic fields in its vicinity which are capable of saturating the core and thus introducing errors in the output signal or cancelling the output. In addition, no provision is provided for cancelling the effects of non-uniform magnetic fields originating external to the measurement device, as are routinely experienced in watthour meters.
In addition to the foregoing set out disadvantages, prior art current sensors, and their attendant current dividers, of the type disclosed in the above patents have been found to suffer from two other principal disadvantages; namely (1) non linearity, and (2) phase shift between the input current and the output current or voltage from the sensor. The cause of items (1) and (2) are explained as follows: